1. Field of the Invention
This invention relates to a mercury free arc tube for a discharge lamp provided with a sealed glass chamber which is filled with at least a metallic halide for main light emission as well as a rare gas by pinch-sealing both end openings of a glass tube and electrode bars are opposite to each other. This invention particularly relates to a mercury free arc tube for a discharge lamp provided with electrode bars each having such a concentric stepped shape that the cross sectional area of a tip side region projecting into the sealed glass chamber is larger than that of a base side region provided in a pinch-sealed portion.
2. Description of the Background Art
FIG. 4 illustrates a related art discharge lamp device. The front end of an arc tube 5 made of quartz glass is supported by a single lead support 2 which projects forward from an insulating base 1. The rear end of the arc tube 5 is supported by a concave portion 1a of the insulating base 1. The area adjacent to the rear end of the arc tube 5 is held by a metallic supporting member 4 secured to the front face of the insulating base 1. The lead wire 8 on the front end side led out from the arc tube 5 is fixed to the lead support 2 by welding. On the other hand, the lead wire 8 on the rear end side passes through a bottom wall 1b on which the concave portion 1a of the base 1 is formed and fixed to a terminal 3 formed on the bottom wall 1b by welding. Symbol G denotes a cylindrical glass globe for cutting off a component of ultraviolet rays which have a wavelength which is harmful to the human body and which are emitted from the arc tube 5. The globe G is integral with to the arc tube 5.
The arc tube 5 has a structure in which between a pair of front and rear pinch-sealed portions 5b, 5b, a sealed glass chamber 5a is formed in which electrode bars 6, 6 are opposite to each other and a light emitting material (halide of Na or Sc and Hg) is sealed with rare gas. Within each of the pinch-sealed portions 5b, a molybdenum foil 7 is deposited for connecting the electrode bar 6 projecting into the sealed glass chamber 5a and the lead wire 8 led out from the pinch-sealed portion 5b, thereby assuring hermeticity of the pinch-sealed portions 5b. 
The mercury (Hg) filled in the sealed glass chamber 5a is a very useful substance to keep a predetermined tube voltage and to reduce the quantity of collisions of electrons with the electrode to thereby alleviate damage of the electrode. However, since Hg is harmful to environment, in recent years, development of a “mercury free arc tube” in which Hg is not contained has been advanced.
In the case of a “mercury free” arc tube, the tube voltage is lowered so that the tube power necessary for discharging cannot be obtained. So, in order to increase the tube electric power, it is necessary to increase the current (tube current) to be supplied to the arc tube. The electrode tip correspondingly reaches a high temperature.
Thus, if ON/OFF of the arc tube is repeated, the crystal in the vicinity of the electrode tip will grow (crystal size will expand) so that the shape of the electrode end face changes owing to shifting of a crystal interface position. Thus, the “decline” of the luminescent spot such as displacement of the luminescent spot (the luminescent spot of discharging shifts whenever the arc tube is turned ON/OFF) or shift of the luminescent spot (the luminescent spot shifts while the arc tube is stably kept “ON”) occurs. This makes it difficult to acquire appropriate distributed light and to reduce the central brightness of a vehicle-use head lamp.
In order to obviate such inconvenience, in related art patent reference JP-A-2004-220880, as the longitudinal cross sectional structure of the tip side region projecting into the sealed glass chamber of the tungsten electrode bar 6 of the mercury free arc tube, proposed is the structure in which the number of crystals residing in a region 6a extending from the tip of the electrode tip to the distance equal to the diameter d of an axial portion is 5 or less and the number of crystals residing in the remaining tip side region 6b is 10 or more, as shown in FIG. 5.
In this configuration, since there is less grain boundary at the electrode tip, there is less changes in the shape of the electrode end face resulting from that the crystal interface position changes owing to crystal growth. So, there is less decline of the luminescent spot during discharging. As a result, there is less change in the distributed light and less reduction in the central brightness in a vehicle-use head lamp.
However, in the JP-A-2004-220880 reference, the longitudinal cross sectional structure of the tip projecting into the sealed glass chamber of the tungsten electrode bar is constructed of five or less crystals (e.g. FIG. 5 illustrates a total of three connected crystals consisting of a large crystal C2 at the center and crystals C1 and C3 on the upper and lower sides). In this way, as long as the cross sectional structure of the tip of the electrode bar is constructed of a plurality of crystals, the decline of the luminescent spot during discharging cannot be surely avoided.
Specifically, where ON/OFF of the arc tube is repeated, the crystals C1, C2, C3 grow (their crystal size expands) so that the crystal interface positions P1, P2 shift. As a result, the shape of the electrode tip changes, thereby leading to the decline of the luminescent spot. Namely, the problem of a change in the distributed light and reduction in the central brightness in a vehicle-use head lamp is not solved.